The present invention relates to an air pressure detection device which directly detects an air pressure of tire by pressure detecting section disposed on each wheel.
Recently, there have been well known that a motor vehicle has a pressure gauge to indicate a pressure decrease to a passenger as soon as the air pressure goes down below a predetermined pressure value. There are two well known devices for that purpose, that is, directly or indirectly to detect the pressure decrease of the respective tires.
The air pressure detection device that directly detects the air pressure in the tire is provided with the pressure detecting section disposed on each wheel to detect the air pressure, and from the pressure detecting section, the pressure value is wirelessly transmitted to a receiving section disposed in the vehicle.
Furthermore, the air pressure detection device that indirectly detects the air pressure of the tire is provided with speed detecting section disposed on a hub of an each axles to detect a rotating speed of the wheel, and from the speed detecting section, the wheel rotating speed is wire-transmitted to a receiving section disposed in the vehicle (for example, refer to JP-A-10-512515) . In this case, by comparing the wheel rotating speeds according to running conditions, the air pressure is calculated.
In both direct-type and indirect-type air pressure detection devices, when at least one of the tires becomes lower in the pressure than a predetermined defective pressure value, by using a display section disposed in the vehicle, the driver is informed of which of the tires has lowered in air pressure. Thereby, before the normal running of the vehicle becomes impossible, the driver can recover the air pressure in question to the desired pressure value.
Herein, the indirect-type air pressure detection device has a problem in that a detection accuracy thereof is inferior to that of the direct type, although the air pressure can be calculated from the wheel speed by using the speed detecting section of an anti-lock braking system, etc. Therefore, when the detection accuracy is important, the direct type air pressure detection device is mounted on a vehicle.
However, in the direct-type air pressure detection device, when the four wheels change positions by a tire rotation, it cannot be recognized which air pressure value of the tire is transmitted to the receiving section. Namely, even when the driver recognizes the air pressure decrease, the driver cannot judge which wheel should be replaced with a spare tire, and as a result, the driver cannot take an emergency measure until the driver arrives at a gas station where the air pressure can be measured. Herein, it is considered that receiving sections are set at positions close to respective wheels to detect separately the air pressure values of the wheels, however, this makes the device expensive.
In order to solve these problems, there is known a method in which a tire positional information is transmitted to the receiving section from the pressure detecting sections in addition to the air pressure values. However, even when this structure is used, the tire positional information of the pressure detecting sections must be changed for every tire rotation, and this is very inconvenient. Herein, if the tire is continuously used without changing the tire positional information at the time of tire rotation, the tire positional information different from real tire positions is transmitted and the driver is informed of an incorrect information.